When examining body fluids, it is often desirable to study samples of such biological material under the microscope over a certain time. When examining sperm, for example, the aim is to establish how many spermatozoa are present in the sample and also how motile they are and how their morphology is. When examining blood samples, for example, the aim is to establish how many red and/or white blood cells are present in the sample and also how their morphology is.
To carry out this investigation, a sample of a certain thickness is to be subjected to microscopic examination in a counting compartment, using a grid built into the eyelens of the microscope. Such a grid may be divided into a hundred squares, and the number of cells (such as spermatozoa or blood cells) in each of a representative number of squares can be counted by the investigator in order to determine the total number of cells in the whole grid area. Such a grid may also be provided in the counting compartment.
From U.S. Pat. Nos. 4,911,782 and 5,200,152 a method is known for conducting such determinations with the aid of a counting compartment formed by two transparent plates joined together by a connecting layer composed of a cured plastic.
From U.S. Pat. No. 6,551,554 B1 and EP patent No. 0 809 815 B1 a counting device is known, which comprises two transparent plates which are held at a fixed distance from each other and joined together by a connecting layer, and at least one counting compartment which is situated between the plates, bounded by the connecting layer and fitted with an inlet and an outlet. The connecting layer contains material particles which are separate from one another and have a size that determines the depth of the counting compartment, which material particles are substantially in contact with the two plates.
For a correct quantitative evaluation of a sample it is required that the correct depth between the two glass plates of the counting device is maintained throughout the complete counting area. It is also required that there is no variation between counting devices in order to allow a correct quantitative evaluation of all samples to be analyzed using such a device. However, this counting device is a one-way device and requires a new counting area for each sample thus raising high requirements for uniformity of the devices. The way of loading the device as well as the quality of the sample can influence the result of such an evaluation. As the two glass plates are firmly attached to each other, the sample must pass through a very small gap into the counting area (typically between 10 and 100 μm), being thus exposed to any kind of blocking or sieving effects developing at such a small gap. The qualitative analysis of a sample of cells like sperm requires a completely inert and non-toxic environment to allow observation of unimpaired motility and morphology of the sperm. Typically counting devices consisting of two glass plates use a glue or adhesive to attach the glass plates to each other. Such binding material often is toxic to sperm and impairs motility and or morphology of the sperm.
There is a need in the art for improved counting devices and methods for analysing samples comprising cells and/or particles, which in particular allow to obtain accurate and improved measurement results.